PAUT & TOFD Weld Inspection Singapore — Advanced Ultrasonic Weld Testing

What Is PAUT & TOFD Weld Inspection?

PAUT and TOFD weld inspection is advanced ultrasonic testing used to detect, size and characterise flaws inside welds — without cutting the joint open and without radiography. Where a conventional ultrasonic probe fires a single beam, these two techniques build a far richer picture of the weld volume, resolving the location, length and through-wall height of each discontinuity so it can be judged against a code acceptance standard. They are the workhorse volumetric NDT methods for welds on process piping, pressure vessels, storage tanks and structural steel across Singapore's process and marine industries.

This is a methods capability that sits alongside — but is distinct from — our UT thickness measurement and corrosion-under-insulation surveys. Thickness gauging tells you how much wall is left; PAUT and TOFD tell you whether the weld holding the equipment together is sound. Both matter, but they answer different questions, and a weld examination demands its own scanners, calibration and interpretation discipline.

The two techniques are complementary and are routinely deployed together. PAUT is exceptional at sweeping the weld body and mapping where flaws sit; TOFD is exceptional at measuring how tall those flaws are and whether they are growing. Run as a pair, they give near-complete coverage of a weld with the accuracy needed for a fitness-for-service decision.

How Phased Array Ultrasonic Testing (PAUT) Works

A phased array probe is not one crystal but an array of many small ultrasonic elements. By pulsing those elements with tiny, precisely controlled time delays, the instrument electronically steers and focuses the ultrasonic beam through a range of angles from a single probe position — no manual re-angling required. The returning echoes are assembled into a detailed cross-sectional image of the weld, so the inspector sees the joint in section rather than interpreting a single deflection on a screen.

That imaging is what makes PAUT so effective on welds. Because the beam sweeps through the full weld bevel, it reliably picks up lack of fusion along the fusion face, incomplete penetration at the root, porosity, slag inclusions and cracks — and it places each one at a real depth and position. Encoded scanning, where the probe runs on a mechanised scanner tracking its position, produces a permanent, repeatable record of the entire weld length, which is essential for comparing results between inspections and for defending acceptance decisions.

How Time-of-Flight Diffraction (TOFD) Works

TOFD takes a different physical approach. Instead of relying on the strong echo reflected back from a flaw face — which depends heavily on flaw orientation — TOFD uses the weak ultrasonic waves that diffract from the tips of a discontinuity. A pair of angled probes, one transmitting and one receiving, straddle the weld. When a flaw is present, signals diffract from its upper and lower tips, and the difference in their arrival times gives a highly accurate measurement of the flaw's through-wall height and depth.

Because it reads flaw tips rather than reflected energy, TOFD is remarkably sensitive to planar defects such as cracks and lack of fusion — precisely the flaws that concentrate stress and drive failures — and it is largely insensitive to how the flaw is oriented. It also excels at flaw growth monitoring: re-scanning the same weld at intervals shows whether a known indication is extending, which is exactly the information a plant needs to plan a repair. In practice TOFD is paired with PAUT so that PAUT's detection and mapping strength covers TOFD's near-surface dead zones, giving full-volume examination.

The Key Advantage: A Radiography Replacement

On a large share of weld inspection work, PAUT and TOFD directly replace conventional radiographic testing (RT), and on a live plant the case for doing so is compelling.

• No ionising radiation — there are no radiation sources on site, so there is no radiation-safety hazard to personnel and none of the licensing and handling burden that comes with gamma or X-ray sources.
• No exclusion zones or night shifts — radiography forces surrounding trades to stop and the area to be cleared, often pushing shots to night shift. Ultrasound needs no evacuation, so inspection runs alongside other work with far less disruption.
• Immediate digital results — there is no film to expose, develop and interpret. Indications are seen and sized on the spot, so accept/reject and repair decisions happen in real time.
• Permanent, auditable records — encoded scans store the full dataset for every weld, giving a traceable record for code acceptance and later fitness-for-service assessment.

The net effect is a method that is both safer and less disruptive than radiography, while delivering superior flaw sizing. That combination is why advanced ultrasonics has become the default weld NDT choice on modern oil and gas construction and turnaround scopes.

Where PAUT & TOFD Are Used

Our advanced ultrasonic weld inspection covers both new-construction and in-service work. On new fabrication, PAUT and TOFD verify weld quality before commissioning as a radiography alternative. In service, they are central to turnaround weld examinations, checking critical joints on piping and vessels while the plant is down. The same phased array technology also drives automated corrosion mapping, running a C-scan across a vessel or tank wall to build a full thickness map of an area rather than a scatter of spot readings.

Scanning is mechanised wherever possible: an encoder tracks probe position so the scan can be replayed and compared weld-to-weld over time. Access to elevated or awkward joints is arranged by rope access, scaffold or, on large vessels, a magnetic crawler carrying the scanner. One honest note on scope — PAUT and TOFD are surface-contact techniques. The scanner must couple to the steel, so this is a hands-on NDT method delivered by a technician, not a free-flying drone survey. It complements our drone-based visual and thermal inspection rather than replacing it.

What We Examine

The technique is applied wherever a critical weld carries pressure, load or containment. Typical scopes include:

Standards & Codes We Work To

Advanced ultrasonic weld inspection is only as good as the procedure behind it. Our examinations are performed and reported against the recognised international codes and standards for the method and the equipment:

Why Choose SG Drone Inspections

We provide a full industrial NDT and inspection capability in Singapore, combining advanced ultrasonics with drone-based visual and thermal survey so an asset can be assessed as a whole.

Frequently Asked Questions